In every consumer device, such as cellular telephones, global positioning devices, two way radios, wireline, TV tuners, disk drives, fiber-optics, and the like, synthesis of a carrier frequency is an essential operation of the transceiver, and many other communication functions. Frequency synthesizers in all high performance cellular transceivers employ Phase-Locked Loop (PLL) as the circuit solution to the challenge of producing a carrier frequency with low noise and high frequency accuracy.
A circuit known as Charge Pump is employed in the majority of PLL synthesizers. Charge pump circuits contain two branches, known as Up and DOWN branches. The currents of two branches are integrated on a capacitor, and the resultant capacitor voltage, with some additional filtering, serves as a control voltage for the Voltage Controlled Oscillator (VCO). Optimal operation occurs when the up and the down currents are equal to each other, but practical circuits will frequently suffer from mismatch between the up and the down circuit. The mismatch can produce a number of undesirable phenomena, including an increase in the reference spurious tone. The imbalance between the up and the down circuits also causes phase-to-voltage transfer function of the Charge-Pump-Phase frequency detector to become nonlinear.
This is undesirable, especially in the case of fractional-N PLLs. The fractional divided of fractional-N PLL produces shaped quantization noise, with peak values of noise occurring near ½ reference frequency. Even-order components of Charge-pump nonlinearity cause substantial component of the noise near ½ reference frequency to be partially downconverted down to the low frequencies near DC. This is known in the industry as noise folding of components near ½ reference frequency. While noise components located near ½ reference frequency are substantially attenuated by the Low pass filter integrated into the charge pump, components downconverted to the low frequencies are unaffected by the low-pass filter and cause deterioration in PLL's noise performance.
Thus, a need still remains for a communication system with charge pump mechanism for synthesis of a carrier frequency. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is increasingly critical that answers be found to these problems. Additionally, the need to reduce costs, improve efficiencies and performance, and meet competitive pressures adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.